Resonant circuit for electronic anti-theft element

ABSTRACT

A resonant circuit for an anti-theft element consists of two spiral printed circuits and one dielectric layer. The spiral printed circuits are wound in opposing directions and arranged on opposite sides of the dielectric layer so that they at least partly overlap. At least one selected area is provided in which a conductive path arises between the two spiral printed circuits whenever a sufficiently high energy is applied by means of an external alternating field.

CROSS REFERENCE TO RELATED APPLICATION

This application discloses subject matter in common with co-pendingapplication, Ser. No. 09/147,646, filed Feb. 8, 1999.

FIELD OF THE INVENTION

The present invention relates to a resonant circuit for electronicarticle surveillance.

BACKGROUND OF THE INVENTION

Resonant circuits which are excited to resonate at a predeterminedresonant frequency which is conventionally at 8.2 MHz are widelyaccepted as anti-pilferage devices in department stores. Frequently thecircuits are an integral part of adhesive labels or cardboard tags whichare affixed to the articles to be maintained under surveillance.Typically, the department store has an electronic surveillance systeminstalled in the exit area, which detects the resonant circuits andproduces an alarm when a protected article passes through a surveillancezone in an unauthorized manner. The resonant circuit is deactivated whena customer has paid the merchandise. This prevents an alarm beingproduced once an article has been rightly acquired by purchase, passingthrough the surveillance zone subsequently.

The deactivation systems which are frequently installed in the checkoutarea generate a resonant signal of a higher amplitude than it isproduced in the surveillance systems. A resonant label is normallydeactivated with a signal whose field strength is greater than 1.5Ampere turns per meter.

A variety of deactivating mechanisms for resonant circuits are known inthe art. They involve either destroying the insulation between twoopposing conductive tracks, producing a short circuit, or subjecting alength of conductive track to overload and causing it to melt, therebyinterrupting the circuit path. As a consequence of deactivation, theresonant properties of the resonant circuit, that is, the resonantfrequency and/or the “Q” factor are modified so severely that theresonant label stops being detected by the surveillance system.

With regard to the deactivation of resonant labels, different methodshave been described in the art. In U.S. Pat. No. 4,876,555 and itscorresponding European Patent, EP 0 285 559 B1, it is proposed to use aneedle to produce a hole in the insulating layer between two oppositecapacitor surfaces. This results in a fault-free and permanentdeactivation mechanism.

U.S. Pat. No. 5,187,466 describes likewise a method for generating adeactivatable resonant circuit by means of a short circuit.

As regards the first mentioned U.S. Pat. No. 4,876,555 and itscorresponding European Patent EP 0 285 559 B1, it should be noted thatthe resonant circuit therein disclosed includes capacitor plates whichare disposed on either side of a dielectric material. The dielectriclayer arranged between the two capacitor plates has a through hole.

In U.S. Pat. No. 5,187,466 referred to in the foregoing, a method isdescribed which is applied to a resonant circuit having capacitor plateson either side of a dielectric, and in which the capacitor plates arefirst short-circuited and the short circuit is melted later by theapplication of electrical energy.

European Patent EP 0 181 327 B1, describes a deactivatable resonantlabel which is composed of a dielectric substrate layer, capacitorplates on either side of the dielectric layer, and a coiled winding onone of the two sides of the dielectric layer. To ensure reliabledeactivation of the resonant label, a selected area is treated fordeactivation. In particular, in this area the dielectric layer isthinner than in the remaining areas.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a resonant circuitwhich can be deactivated reliably.

This object is accomplished in that the resonant circuit is comprised oftwo coiled conductive tracks and one dielectric layer, said twoconductive tracks being wound in opposite directions and disposed oneither side of the dielectric layer such as to overlap at least in part,with at least one selected area being provided in which a conductivepath is produced between the two tracks as soon as energy in asufficiently high amount is applied by an external alternating field.Thus the present invention has no separate capacitor plates; rather,these are formed directly by the two at least partly overlapping tracks.

According to an advantageous further aspect of the resonant circuit ofthe present invention, the dielectric layer is of substantially uniformthickness and has no additional manufacturing defects (air inclusions,for example).

This configuration is particularly advantageous in combination with ayet further aspect according to which the selected area is at the outerend areas of the tracks where the tracks' induced voltage is at itshighest level. Any special treatment of any point on the resonantcircuit is thus entirely superfluous with this configuration. Utilizingthe laws of physics, the deactivation area is automatically in apredeterminable area at the outer ends of the coiled tracks.

In an alternative configuration of the resonant circuit of the presentinvention it is proposed that the selected area be at any desired pointon the overlapping tracks and be treated such that the conductive pathis built up at the point thus treated when the deactivation signal isapplied.

In this connection particularly, provision is made for the dielectriclayer to be thinner in the selected area than it is in the remainingareas, or for the treated point to be a hole in the dielectric layer. Ina further configuration of the resonant circuit of the presentinvention, provision is made for the dielectric layer to have adifferent physical or chemical property in the selected area.

In an advantageous further aspect of the resonant circuit of the presentinvention, the dielectric layer is comprised of at least two components.This enables dielectric layers to be produced which are highlyhomogeneous and contain air inclusions in negligible amounts only. Inthis configuration, therefore, it has proven to be advantageous for themelting point of the one component to lie above the productiontemperature for the resonant circuits, that is, this layer will not meltduring the manufacturing process. According to a further aspect of theresonant circuit, the components are furthermore of a nature enablingthem to be joined together by either a coating or a laminating process.

Reference has been made in the foregoing to the advantageous embodimentof the resonant circuit of the present invention in which thedeactivation area occurs in the overlapping outer end areas of thecoiled tracks on account of physical conditions. To enhance this effectstill further, in an advantageous further aspect of the resonant circuitof the present invention, the areas of overlap between the two tracks,and hence the capacitance between the coiled tracks, are concentrated atthe inner ends of the tracks.

It is therefore a further object of the present invention to provide adielectric layer which is substantially uniform in thickness and largelyfree from local weak points occurring in production. Such a uniformdielectric layer 4 ensures deactivation at those points where voltageand energy are at their highest levels, that is, related to the exampleshown at the ends of the upper track 3. Short circuits produced by suchdeactivation are very robust with little susceptibility to accidentalreactivation.

Furthermore, reliability of deactivation can be improved still furtherby arranging for the outer ends of the two tracks to overlap in a smallarea and by having a relatively long length of track with no overlapadjacent to the outer ends of the tracks.

The present invention will be explained in more detail in the followingwith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a plan view of an embodiment of the resonant circuit of thepresent invention;

FIG. 1 b is a side view taken in the direction of arrow A of FIG. 1 a.

FIG. 2 is a cross sectional view of the resonant circuit of FIG. 1 takenalong the line II—II of FIG. 1 a.

FIG. 3 is a schematic illustration of the voltages with two partlyoverlapping coiled tracks;

FIG. 4 is a plan view of the outer end area of the coiled tracks;

FIG. 5 is an enlarged cross sectional view of the upper coil and theupper component of the dielectric layer; and

FIG. 6 is a detailed cross-sectional view of the resonant circuit of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 a and 1 b show an embodiment of the resonant circuit 6 of thepresent invention on a substrate material 1 in plan view, and in a sideview, respectively. FIG. 2 shows the resonant circuit 6 of FIG. 1 incross sectional view.

Deactivation of the resonant circuit 6 takes place by producing a shortcircuit between the two coiled conductive tracks 2, 3, through thedielectric layer 4. The two coiled conductive tracks are preferablyfabricated from aluminum. The application of an alternating magneticfield as emitted, for example, by the surveillance system inducesalternating voltages in the two coiled tracks 2, 3 of the resonantcircuit 6. The two coiled tracks 2, 3 which overlap at least in part arewound in opposite directions. Therefore, the outer end of the lower coil2 has a positive potential with respect to the inner end of the lowercoil 2 when the inner end of the upper coil 3 has a positive potentialwith respect to the outer end of the upper coil 3. It will beunderstood, therefore, that the points/areas in which the inducedalternating voltages between the two coils 2, 3 are at their highestlevels are located in the end areas of the coils 2, 3. The point ofoverlap is adjacent a relatively long length of track (9 in FIG. 1 a)having no overlap.

Considering that in the example illustrated in FIG. 1 the upper coil 3has fewer turns than the lower coil 2, the highest voltages aregenerated between the ends of the upper coil 3 and the areas of thelower coil 2 situated directly underneath. FIG. 3 illustrates clearlythe voltage relationships in different areas of the two at least partlyoverlapping coils 2, 3 of a resonant circuit 6 that is suitable for useaccording to an advantageous further aspect of the resonant circuit 6 ofthe present invention.

FIG. 3 illustrates the individual voltages occurring in different areasof the two overlapping coils 2, 3 along their length duringelectromagnetic induction.

In the resonant circuit 6 previously described in which the dielectriclayer 4 between the coils 2, 3 is of uniform thickness, deactivationtakes place in the end areas of the upper coil 3 and the lower coil 2,because this is where the induced potential is at its highest level.Because the electric field strength is focused on a surface with a smallradius, deactivation takes place precisely at the ends of the tracks 2,3, as shown in FIG. 4. The dielectric layer 4 may be thinner at thispoint (as seen at 8 in FIG. 1 b) to enhance deactivation.

If however the dielectric layer 4 is not of uniform density or containsair inclusions 7, which may happen easily as a result of manufacturingdefects, deactivation may take place in various areas of the coils 2, 3.Such manufacturing defects may cause local weaknesses and even produceholes resulting from air inclusions in the dielectric layer 4. As aconsequence, the dielectric layer 4 breaks down at these local weakpoints although the voltage potential is lower at these points than itis at the ends of the upper and lower track 3, 2. Because the voltagepotential is lower at the local weak points than it is at the ends ofthe tracks 2, 3, the electrical energy available for producing thedeactivation short circuit is smaller than the electrical energy thatwould be necessary to produce a deactivation short circuit at the endsof the upper coil 3.

FIG. 5 shows a cross section of a dielectric layer 4 exhibitingmanufacturing defects in the form of air inclusions 7 and irregularitiesin the surface area.

According to an advantageous further aspect of the resonant circuit 6 ofthe present invention, the dielectric layer 4 is comprised of at leasttwo components 4 a, 4 b, including an upper component 4 a and a lowercomponent 4 b. The lower component 4 b is applied to the lower coil 2prior to stamping and hot embossing. The upper component 4 a is appliedto the upper coil 3. The upper component 4 a has a relatively lowmelting point enabling it to serve as a hot-melt-type adhesive and toadhesively bond the two coils 2, 3 together during hot embossing of theupper coil 3 onto the lower coil 2. The upper component 4 a of thedielectric layer 4 melts during hot embossing of the upper coil 3.Having a higher melting point, the lower component 4 b of the dielectriclayer 4 does not melt during hot embossing on the upper coil 3. Theuniformity of the lower component 4 b of the dielectric layer 4 whichdoes not melt improves overall the uniformity of thickness of thedielectric layer 4.

FIG. 6 shows a cross section of a resonant circuit 6 having a dielectriclayer 4 composed of two components 4 a, 4 b. The lower component 4 b maybe produced either by coating the lower coil 2 or by laminating thelower component 4 b of the dielectric layer 4 onto the coil 2. Typicallythe coil material (aluminum) is available in the form of broad coilsenabling uniformity of the surface of the dielectric layer 4 to bemaintained and other defects caused, for example, by air inclusions 7,to be minimized.

What is claimed is:
 1. A resonant circuit for electric articlesurveillance, comprising: two coiled conductive tracks and onedielectric layer, wherein said two coiled conductive tracks are wound inopposite directions and are disposed on respective opposite sides of thedielectric layer, such as to overlap at least in part, wherein at leastone selected area is provided in which a conductive path is producedbetween the two coiled conductive tracks as soon as energy in asufficiently high amount is applied by an external alternating field,and wherein the dielectric layer is of substantially uniform density. 2.The resonant circuit as claimed in claim 1, wherein the dielectric layeris comprised of at least two components.
 3. The resonant circuit asclaimed in claim 2, wherein the melting point of one of the componentslies above the production temperature for the resonant circuits.
 4. Theresonant circuit as claimed in claim 2, wherein the components are of anature enabling them to be joined together by either a coating or alaminating process.
 5. The resonant circuit as claimed in claim 1,wherein the areas of overlap between the two coiled conductive tracksand hence the capacitance between the coiled conductive tracks areconcentrated at the inner ends of the coiled tracks.
 6. The resonantcircuit as claimed in claim 5, wherein the outer ends of the two coiledconductive tracks overlap in a small area and a relatively long lengthof track with no overlap is adjacent to the outer ends of the coiledconductive tracks.
 7. The resonant circuit as claimed in claim 1,wherein the selected area is characterized in that the dielectric layerhas a hole.
 8. A resonant circuit for electronic article surveillance,comprising: two coiled conductive tracks and one dielectric layer,wherein said two coiled conductive tracks are wound in oppositedirections and are disposed on respective opposite sides of thedielectric layer, such as to overlap at least in part, wherein at leastone selected area is provided at the outer end area of one of said twocoiled conductive tracks and the overlapping areas of the said twocoiled conductive tracks are situated directly underneath where theinduced voltage of said one of said two coiled conductive tracks is atits highest level, and wherein a conductive path is produced in theselected area between said two coiled conductive tracks as soon asenergy in a sufficiently high amount is applied by an externalalternating field.
 9. A resonant circuit for electronic articlesurveillance, comprising: two coiled conductive tracks and onedielectric layer, wherein said two coiled conductive tracks are wound inopposite directions and are disposed on respective opposite sides of thedielectric layer, such as to overlap at least in part, wherein at leastone selected area is provided in which a conductive path is producedbetween the two coiled conductive tracks as soon as energy in asufficiently high amount is applied by an external alternating field,the selected area is treated such that the conductive path is built upin the selected area when the deactivation signal is applied, andwherein the dielectric layer is thinner in the selected area than it isin the remaining areas.
 10. A resonant circuit for electronic articlesurveillance, comprising: two coiled conductive tracks and onedielectric layer, wherein said two coiled conductive tracks are wound inopposite directions and are disposed on respective opposite sides of thedielectric layer, such as to overlap at least in part, wherein at leastone selected area is provided in which a conductive path is producedbetween said two coiled conductive tracks as soon as energy in asufficiently high amount is applied by an external alternating field,the selected area is treated such that the conductive path is built upin the selected area when the deactivation signal is applied, andwherein the dielectric layer has a different physical or chemicalproperty in the selected area.